Shopping for a remanufactured caliper, booster, or master cylinder, you will see a "core charge" added to the price. It is simple once you understand it, and you usually get it back. Here is what a core charge is.
A refundable deposit
A core charge is a deposit on the old part (the core). The remanufacturing supply chain runs on returned cores - your worn-out unit becomes the basis for the next reman part; see reman vs new. The charge is refunded when the old unit comes back.
How it works in practice
You pay the part price plus the core charge up front. After you install the new part, you send back the old one, and the core charge is refunded. It is not an extra cost in the end - it is a deposit that encourages returning the recyclable core.
Why it exists
Cores are valuable - the metal casting is the expensive part to make. The core system keeps good castings in circulation and keeps reman parts cheaper than new. It is standard across the industry for hydraulic and electronic parts.
The bottom line
Do not let a core charge scare you off a reman part - it is money you get back. Reman calipers and boosters are a smart value; see loaded caliper. Pair a fresh part with quality pads and hardware.